You can find a lot of examples on how to do snap to screen on our documentation. I put just one of them here. In addition, if you want to add the social media share functionality, I would advice you to follow the example here regarding how to add interactivity in your app and then take care of the part where you have to add the code for sharing on Facebook.
I'd recommend Acethinker Screen Grabber which I have used for many years, It's a web-based application to capture screenshot, you don't have to download or install anything. It comes with a social media uploader, you can upload the screenshot to facebook, twitter and Google+ directly.
How to capture screen then share to social media such as Facebook?
is there any example?